Controlling the relative humidity level, whether in an aircraft or in a building, is desirable in order to maintain a comfortable environment for the people within. If the relative humidity in an aircraft is too low, generally below 10%, passengers become uncomfortable due to the occurrence in a long duration flight of, for example, dry, itchy skin, nasal irritation, and gritty eyes. This condition of low relative humidity naturally occurs in modern, jet-engine powered commercial aircraft which typically use bleed air from the engines and/or the auxiliary power unit (APU) as the source of air for pressurization. The bleed air from the various compressor stages of the engines are interconnected by bleed air ducts and control valves and fed to as many as three separate air conditioning systems. The air exiting the air conditioning systems is mixed in a plenum chamber and distributed to the passage compartment by ducting mounted above the passenger compartment. The air circulates through the passenger compartment, down through the below-deck cargo compartment walls and then out flow control valves mounted in the bottom of the fuselage. The use of such a system at altitudes above 25,000 ft., however, reduces the relative humidity in the passenger compartment to a value of about 5-7%, causing the uncomfortable conditions mentioned above.
In order to rectify this condition, moisture must be introduced into the above air conditioning system. While a 50% relative humidity level is considered to be ideal for passenger comfort, levels of 15-30%, still adequate to ensure passenger comfort even on flights as long as 9 to 11 hours, are desirable to reduce the amount of water that must be carried on board the aircraft to raise the humidity, as such additional water adds weight and therefore increases fuel consumption and reduces payload. Additionally, maintaining the relative humidity at between 15-30% reduces the possibility of condensation of moisture on cold surfaces, thus reducing the possibility of corrosion of the aircraft structure, and condensation of moisture on the overhead structure of the passenger compartment, thus reducing the possibility of water droplets forming which can fall on the passengers and crew.
The prior art humidification systems designed for use in aircraft, such as mechanical atomizers or centrifugal "slingers", inject droplets of water into the temperature-conditioned air. These systems do not, however, ensure that the water is completely vaporized. Therefore, water tends to collect on ducting surfaces within the aircraft with the results previously mentioned. If the injection nozzle apparatus of these devices is made small enough to more completely atomize the water, calcium and other mineral deposits rapidly build up and clog the nozzle. They thus must be cleaned often, causing higher maintenance costs. These mineral deposits are also carried into the flight station and electronic bays causing corrosion. While such contamination can be avoided by using distilled water, this increases operating costs and is not practical for commercial aircraft.
Most modern humidification systems, furthermore, as shown, for example, in U.S. Pat. No. 3,642,201, "Humidifier Control", by P. E. Potchen, require humidity level sensors in order to control the humidity level. Airlines are reluctant, however, to install such complicated devices on board their aircraft and prefer open-loop systems.
Accordingly, it is a general object of the present invention to provide a humidification system for humidifying a compartment of an aircraft.
It is another object of the present invention to provide a humidification system for an aircraft that is capable of maintaining a substantially constant level of humidity using a source of air that varies in temperature and pressure, such as bleed air from the engines.
It is a further object of the present invention to provide a humidification system for an aircraft that minimizes the usage of bleed air.
It is another object of the present invention to provide a humidification system that completely vaporizes injected water and is tolerant of mineral deposits so that tap water can be used.
It is still another object of the present invention to provide a humidification system that will control the humidity level within desired limits without the use of a humidity sensor.